A letter from four nuclear scientists who contend the government released classified H-bomb concepts in pursuing the Progressive magazine case has itself been classified by the Deparment of Energy.

The letter from the scientists, all of whom work at the Argonne National Laboratory near Chicago, was sent to Sen. John Glenn (D-Ohio) April 25. It called for a congressional investigation of "several potentially sensitive affidavits" made public by the government in its lawsuit to prevent the Progressive from publishing an article about the H-bomb.

In indentifying what they believe to be government-released classified information, the scientists specified secret data, according to DOE officials.

"It's ironic," a DOE official said yesterday of the department's action, which in effect accuses the scientists of doing exactly what the scientists charged the government with doing.

"We are not involved in a cover-up." the DOE official said. "We got caught between a rock and a hard place and are trying to protect [classified material] the best we can."

The letter, a copy of which was sent to The Washington Post in late April, identifies what it describes as publicly available material and quotations from government affidavits in the Progressive case. The scientists cited these to support their allegations that "the documents released by the government collectively identify the design concept on which U.S. thermonuclear weapons are based, and reveal that this design concept is far superior to all other known configurations."

DOE classified the letter for two reasons, an officials said.

One reason, which a congressional source called the "derivative theory," is based on the idea that someone who knows the truth of a classified matter, such as H-bomb design concepts cannot, without violating security rules, point out materail in the public domain - such as an encyclopedia article - and say it correctly portrays the secret.

The scientist's letter, according to informed sources, by specifying certain facts about an article and diagram on the H-bomb by Dr. Edward Teller in the 1976 Encyclopedia Americana, revealed secret information.

The second reason is that the scientists' letter according to DOE officials, went beyond material contained in the publicly released affidavits and added informations not even contained in the uncensored affidavits.

For instance, an affidavit by Jack Rosengren, a former associate director for nuclear weapons design of the Lawrence Livermore Laboratory, was cited in the letter as stating that a "diagram" in the Progressive article revealed "the basic design concept(s)" of U.S. H-bombs.

The publicly released Rosengren affidavit, however, makes no reference to a diagram, only to the article itself.

The diagram reference is important to the point the scientists then make in their letter. They say - though Rosengren Didn't - that the article's diagram is the equal of the Teller encyclopedia diagram, thereby implying that a publicly available diagram is the same as the still-secret article diagram.

As one government source said, the letter at times "constructs an inevitable chain of logic that is not that invevitable."

A DOE official said yesterday that the scientists' letter was received from Glenn's office earlier this week. After reviewing it, DOE security officials determined the letter contained secret data under terms of the Atomic Energy Act of 1954.

A copy of the letter also was filed Wednesday in the Progressive case by the American Civil Liberties Union as part of the material supporting its motion to have the injunction on publication of the H-bomb article lifted.

Copies of the letter also were sent to several other members of Congress.

Gelnn's subcommittee has placed its copy in a safe. The House Government information subcommittee, an other recipient, when called by the DOE and told the letter was classified, asked for a paper explaining the situation.

The four scientists who wrote the letter and the ACLU were informed Thursday that DOE considered it a classified document. The scientists gave DOE the names of those who had been sent copies.

Yesterday, a DOE official "informally" asked The Post not to publish the letter, but no official attempt was made to retrieve it.

The letter, one official said, "is not nearly as bad as" the Progressive article in giving away secrets and nowhere near the importance of the recent incident involving the declassfication of a paper, stored at the Los Alamos Scientific Laboratory in New Mexico, describing the H-Bomb trigger mechanism.

One congressional source close to the matter said Thursday that DOE cannot "keep the lid on" the H-bomb material and added, "I think the whole thing is going to explode in their face."

Walter Pincus reported on intelligence, defense and foreign policy for The Washington Post. He first came to the paper in 1966 and has covered numerous subjects, including nuclear weapons and arms control, politics and congressional investigations. He was among Post reporters awarded the 2002 Pulitzer Prize for national reporting.